The Remarkable Afterglow of GRB 061007: Implications for Optical Flashes and GRB Fireballs

We present a multiwavelength analysis of Swift GRB 061007. The 2-m roboticFaulkes Telescope South (FTS) began observing 137 s after the onset of thegamma-ray emission, when the optical counterpart was already decaying from R10.3 mag, and continued observing for the next 5.5 hours. These observationsbegin during the final gamma-ray flare and continue through and beyond a long,soft tail of gamma-ray emission whose flux shows an underlying simple power-lawdecay identical to that seen at optical and X-ray wavelengths, with temporalslope alpha 1.7 This remarkably simple decay in all of these bands is rare forSwift bursts, which often show much more complex light curves. We suggest theafterglow emission begins as early as 30-100 s and is contemporaneous with theon-going variable prompt emission from the central engine, but originates froma physically distinct region dominated by the forward shock. The afterglowcontinues unabated until at least 10^5 s showing no evidence of a break. Theobserved multiwavelength evolution of GRB 061007 is explained by an expandingfireball whose optical, X-ray and late-time gamma-ray emission is dominated byemission from a forward shock with typical synchrotron frequency, nu_m, that isalready below the optical band as early as t=137 s and a cooling frequency,nu_c, above the X-ray band to at least t=10^5 s. In contrast, the typicalfrequency of the reverse shock lies in the radio band at early time. We suggestthat the unexpected lack of bright optical flashes from the majority of SwiftGRBs may be explained with a low nu_m originating from small microphysicsparameters, epsilon_e and epsilon_B. (Abridged)Comment: Accepted for publication in ApJ; 23 pages including 3 tables and 3 figures; minor amendement